Glass formation in hybrid metal halides via breaking molecular rotational order

The critical progression of structural disorder, which governs the bulk glass-forming ability (GFA), can be elucidated as an ergodicity-breaking process. Understanding the atomic characteristics involved is imperative for establishing advanced glass design principles. However, conventional glasses present significant challenges due to their inherently complex and ambiguous disorder motifs, such as intricate and random atomic clusters or ring distributions. In this work, we synthesize a family of zero-dimensional hybrid metal halides with tunable short- to medium-range structural arrangements to form glasses with diverse GFAs. Through altering molecular shape and surface electrostatic potential of organic cations, their rotational order is able to be broken in a controlled manner. This leads to distinct phase space partitions of molecular movements, allowing for tunable GFAs. Our findings provide a fundamental design approach for synthesizing property-oriented glasses by controlling molecular rotational order, which can be applicable to a wide range of molecular glasses and amorphous solids. Revealing the structural features governing ergodicity breaking is critical to understanding glass formation. Here, the authors synthesise a family of hybrid metal halide glasses, and show that the molecular shape and polarity determine rotational disorder, enabling diverse glass-forming abilities.